It is well established that the ability to vary the extent and duration of opening of an engine inlet valve offers clear advantages in terms of engine performance and efficiency. For example, it is often desirable to implement a higher valve lift when the engine is running at a relatively high speed in order to maximise performance, while lowering the valve lift at lower engine speeds so as to improve efficiency. Similarly, holding the inlet valve open for longer at high engine speeds further increases performance.
Mechanical variable valve lift systems are known, in which a pair of cam sets is provided to drive the inlet valves; one cam set optimised for providing low valve lift at low engine speeds, and the other for providing higher valve lift at higher engine speeds. The vehicle switches between cam sets when an engine speed threshold is passed. This type of arrangement is often referred to as ‘cam profile switching’. A drawback with such arrangements is that the switching creates a step change in engine output, resulting in reduced refinement. Even if the switch is timed to coincide with a crossing point for respective torque curves of each of the cam sets, there can be a momentary loss of performance during the switch.
For this reason, various continuously variable valve lift (CVVL) systems have been proposed, and many are in use in modern vehicles. These systems are able to control valve lift to any desired level within the operational range of the valve. This means that valve lift can be optimised throughout the engine speed range, thereby enabling true optimisation of engine performance.
One such arrangement employs a hydraulic system for controlling valve lift. While this system is very effective under normal operating conditions, a problem has been identified where the engine is started in very cold conditions, for example at a temperature of below −10° C. This is because the viscosity of oil used as hydraulic fluid rises significantly at such low temperatures, to the extent that the responsiveness of the system is reduced. A particular problem arising from this is that the time taken for the valve to close is extended, leading to a loss of torque when the inlet valve remains open during part of a compression stroke of a respective engine cylinder.
It is against this background that the present invention has been devised.